Ask to join.

"Mind if I join?" you ask now that they have finally noticed you.
"Lindsey, who's this?" the red head asks as she starts to blush from embarresement. "I thought you said it would just be us."
"Ashley, this is my room mate Chaz. I didn't know he would be home so early."
"We'll I came home early to see you. I needed a good fuck." you say.
"I came her to have my pussy ate not my hymen breached." says the red head.
"Come on Ashely, he doesn't have to take your virginity. Wouldn't you say that oral job I gave you was worth getting fucked in the ass?" begs Lindsey.
"Well I don't know, I've never had it up the ass before. Will it hurt?"
"Of course not" Lindsey says slyly,"You'll be gentle, won't you Chaz?"
"Sure" you say "I'll be as gentle as a lamb."
"Well, ok." agrees Ashley "but, only if Lindsey eats me out again tommorrow."
"It's a deal" agrees Lindsey as she reaches for you cock.